Okay, here's something to stump you guys
 

OKay, here is a stumper for all of you technical types...

1978 W123 300D

Please explain why the cruise control on this car works perfectly at temperatures above 85 degrees (air temp.), intermittently between 75 and 84 and not at all below 75. Honestly, this is the truth. I've watched this thing do this now for two years. Any ideas? My MB mechanic would just like to replace everything...

Cheers

Jeffrey

My 300D had the exact same characteristics. I repaired it by replacing the cc module with a rebuilt one. On my car the module is located underneath the steering wheel above the kick panel - it is an aluminum "box" a little larger than a pack of cigs. I am not sure if your year model has the same one.
Other post - the engine temps you are seeing sound about right.